userdiff: add tests for Fortran xfuncname regex

The Fortran userdiff patterns, introduced in 909a5494f8 (userdiff.c: add
builtin fortran regex patterns, 2010-09-10), predate the test
infrastructure for xfuncname patterns, introduced in bfa7d01413 (t4018:
an infrastructure to test hunk headers, 2014-03-21).

Add tests for the Fortran xfuncname patterns. The test
't/t4018/fortran-comment-keyword' documents a shortcoming of the regex
that is fixed in a subsequent commit.

While at it, add descriptive comments for the different parts of the
regex.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018/fortran-block-data

@@ -0,0 +1,5 @@
+       BLOCK DATA RIGHT
+       
+       COMMON /B/ C, ChangeMe
+       DATA C, ChangeMe  / 2.0, 6.0 / 
+       END 

t/t4018/fortran-comment

@@ -0,0 +1,13 @@
+      module a
+
+      contains
+
+      ! subroutine wrong
+      subroutine RIGHT
+      ! subroutine wrong
+
+      real ChangeMe
+
+      end subroutine RIGHT
+
+      end module a

t/t4018/fortran-comment-keyword

@@ -0,0 +1,15 @@
+      module a
+
+      contains
+
+      subroutine RIGHT (funcA, funcB)
+
+      real funcA  ! grid function a
+      real funcB  ! grid function b
+
+      real ChangeMe
+      integer broken
+
+      end subroutine RIGHT
+
+      end module a

t/t4018/fortran-comment-legacy

@@ -0,0 +1,13 @@
+      module a
+
+      contains
+
+C subroutine wrong
+      subroutine RIGHT
+C subroutine wrong
+
+      real ChangeMe
+
+      end subroutine RIGHT
+
+      end module a

t/t4018/fortran-comment-legacy-star

@@ -0,0 +1,13 @@
+      module a
+
+      contains
+
+* subroutine wrong
+      subroutine RIGHT
+* subroutine wrong
+
+      real ChangeMe
+
+      end subroutine RIGHT
+
+      end module a

t/t4018/fortran-external-function

@@ -0,0 +1,9 @@
+function RIGHT(a, b) result(c)
+
+integer, intent(in) :: ChangeMe
+integer, intent(in) :: b
+integer, intent(out) :: c
+
+c = a+b
+
+end function RIGHT

t/t4018/fortran-external-subroutine

@@ -0,0 +1,5 @@
+subroutine RIGHT
+
+real ChangeMe
+
+end subroutine RIGHT

t/t4018/fortran-module

@@ -0,0 +1,5 @@
+module RIGHT
+
+use ChangeMe
+
+end module RIGHT

t/t4018/fortran-module-procedure

@@ -0,0 +1,13 @@
+ module RIGHT
+
+   implicit none
+   private
+
+   interface letters  ! generic interface
+      module procedure aaaa, &
+                       bbbb, &
+                       ChangeMe, &
+                       dddd
+   end interface
+   
+end module RIGHT

t/t4018/fortran-program

@@ -0,0 +1,5 @@
+program RIGHT
+
+call ChangeMe
+
+end program RIGHT

userdiff.c

@@ -46,9 +46,13 @@ PATTERNS("elixir",
 	 /* Not real operators, but should be grouped */
 	 "|:?%[A-Za-z0-9_.]\\{\\}?"),
 IPATTERN("fortran",
+	 /* Don't match comment lines */
 	 "!^([C*]|[ \t]*!)\n"
+	 /* Don't match 'module procedure' lines */
 	 "!^[ \t]*MODULE[ \t]+PROCEDURE[ \t]\n"
+	 /* Program, module, block data */
 	 "^[ \t]*((END[ \t]+)?(PROGRAM|MODULE|BLOCK[ \t]+DATA"
+		/* Subroutines and functions */
 		"|([^'\" \t]+[ \t]+)*(SUBROUTINE|FUNCTION))[ \t]+[A-Z].*)$",
 	 /* -- */
 	 "[a-zA-Z][a-zA-Z0-9_]*"